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At an unknown point in time before the events of Chapter 4 of The New Colossus, Gruber was assigned to the garrison of the Area 52 facility near Roswell, New Mexico. He was in contact with members of the film industry, having made a recommendation to one of the writers of Das Ende Alles Bösen to try a milkshake (preferably strawberry flavored) while in America.

On July 4 1961, during the Victory Day celebrations in Roswell, Gruber took part in observing the military parade and posed for photos for members of the crowd before heading to Papa Joe's All American Diner for a drink. Gruber's boisterous entrance and the grim connotations of a Nazi commandant's uniform prompted a mother and child in the diner to leave in a hurry; he patted the boy's head, handed the woman her dropped bag and waved them off but made no attempt to apologise for scaring them.

After some chatter with the chef behind the counter and reminding him to translate the menu to German before Changeover Day, Gruber sat at the counter drinking a strawberry milkshake (his "favourite American thing") before interrogating a quiet fireman also present in the diner. Although he found the man's face familiar, he failed to realize that he was sat only a meter or so away from quite possibly the most dangerous man in the world. Asking what station the fireman worked at, he was cut off by the chef giving him his milkshake. Gruber then asked to see the fireman's papers, leading to one of the possible conclusions described below.

If the player does not actively attack Gruber, the officer is initially friendly but soon asks some rather perilous questions about an irregular detail on B.J's otherwise authentic-looking identification papers. Just as B.J. is going to have to formulate an answer, Gruber is interrupted by a communication from the Oberkommando telling him that he's needed immediately. Finishing his milkshake, he turns to leave the diner, but notices a wanted poster of B.J. and realises who "John Smith" really is. He panics and attempts to pull his gun but is killed by Caldwell before he can get a shot off himself.

If the player chooses to attack Gruber before the sequence of events described above has finished, B.J. suddenly and fatally strikes him in the head with the portable atom bomb disguised as a fire extinguisher.

The actor who portrayed Gruber, Heiko Obermoller, also voiced Friedrich Keller, who shares some similar qualities. Notably, they both die through losing their heads - Keller's being cut off and Gruber's being blown apart by either gunfire or blunt trauma.

Gruber's personality and appearance are both clearly influenced by Major Hellstrom, portrayed by August Diehl in the movie Inglourious Basterds. Like Blazkowicz's resemblance to Hugo Stiglitz, this is likely deliberately obvious given Wolfenstein II's influences from the "grindhouse" genre of cinema that also influences Inglorious and other films by Quentin Tarantino.

He is credited as the Milkshake Nazi. Fans on some online forums took to referring to him as "officer milschshake" or "Kommandant erdbeermilsch", but neither nickname is official.

Gruber's name is confirmed by an e-post found inside the Oberkommando, wherein one Vollrath Brotz writes that Gruber was supposed to bring back food from the diner to his post at the Oberkommando; due to his apparent tardiness, Brotz is left hungry.